1. Field of the Invention
The present invention relates to liquid crystal display devices, and in particular relates to fringe field switching mode liquid crystal display (FFS-LCD) devices.
2. Description of the Related Art Liquid crystal display (LCD) devices possess the advantages of having a small size, being light weight and requiring low power consumption. Thus, LCD devices are being applied in a wide variety of electronic and communication devices including notebook computers, personal digital assistants (PDA), and mobile phones. Critical features for large-scale monitors and high-end TV applications include a fast response time, a high contrast ratio, a high transparency, and a wide viewing angle without gray scale inversion. In-plane switching (IPS) mode liquid crystal display devices meet the above-mentioned high quality display feature requirements and solve the viewing angle problems by orienting the liquid crystal molecules to be parallel with a substrate.
Meanwhile, fringe field switching liquid crystal display (FFS-LCD) devices have pixel and counter electrodes comprising transparent conductors and a narrower distance between electrodes than the distance between the upper and lower substrates to form a fringe field on the electrodes. During operation, the fringe field on the electrodes forces the substantially homogeneous liquid crystal molecules to rotate transversely between the substrates in which a wide viewing angle is accomplished since the light is transmitted through the horizontally arranged liquid crystal molecules. Moreover, since the counter electrode and the pixel electrode comprise transparent conductive materials, the aperture ratio and the transmittance ratio of the display devices can thereby be improved.
U.S. Pat. No. 6,856,371, the entirety of which is hereby incorporated by reference, disclose electrode structures of a conventional FFS-LCD device. The electrode structures are symmetrical and render high image display quality and a high transmittance ratio.
FIG. 1 is a cross section of a conventional fringe field switching liquid crystal display (FFS-LCD) device. An FFS-LCD 1 comprises a lower substrate 10, an upper substrate 20, and a liquid crystal layer 30 interposed between the lower substrate 10 and the upper substrate 20, serving as an LCD cell. A counter electrode 11 and a plurality of pixel electrodes 13 are disposed on the lower substrate 10. An insulating layer 15 is disposed between the counter electrode 11 and the plurality of pixel electrodes 13. A lower alignment layer 14 is disposed on the insulating layer 15 and covers the pixel electrodes 13. A color filter layer 25 and an upper alignment layer 24 are disposed on the inner surface of the upper substrate 20 and are adjusted to the liquid crystal layer 30.
FIG. 2 is a plan view of the lower substrate structure of a conventional fringe field switching liquid crystal display (FFS-LCD) device. Two parallel lines 3, 8 and two parallel data lines 7 are orthogonally intersected, enclosing a pixel area. The line 3 is a gate line. A counter electrode 11 and pixel electrodes 13 are disposed in the pixel area. The pixel electrodes 13 comprise two electrode bars 13a parallel to the data lines 7 and a plurality of inclined electrodes 13b with an inclined angle φ between the line m′ and the line m that is parallel to the lines 3, 8. The two ends of each electrode 13b are separately connected to the two electrode bars 13a. Note that the inclined angle φ of the electrodes 13b directly affects the operating voltage of the FFS-LCD device. More specifically, the greater the inclination of electrodes 13b, the higher the voltage required to operate the FFS-LCD device.
For small FFS-LCD panels, the inclined angle φ of the electrodes 13b must be reduced to lower the operating voltage of the FFS-LCD device. A low inclined angle φ of electrodes 13b (e.g., less than 7°) can cause the disclination effect deteriorating display image quality. Conversely, high inclined angle φ of the electrodes 13b requires a high driving voltage such that the physical area of the thin film transistor (TFT) must be increased to provide adequate charge storage capability. The TFT structure comprises a gate line 3, a channel and source/drain regions 4, and source contact 6a and drain contact 6b. The drain contact 6b connects the pixel electrodes 13 via a contact plug 9. When the physical area of the thin film transistor (TFT) increases, however, the area of the pixel electrodes 13 must be reduced, thus, a small aperture ratio and a low transmittance ratio occur.
Thus, FFS-LCD devices with improved aperture and transmittance ratios are desirable.